References known to the Applicants and believed to be relevant to the present invention are U.S. Pat. No. 3,605,863 issued to King on Sept. 20, 1971 and U.S. Pat. No. 4,289,571 issued to Jewett on Sept. 15, 1981. These two patents are hereby incorporated by reference for their teaching of apparatus and methods for formation of ribbon materials which are generally applicable to formation of semiconductor ribbons. The present invention includes apparatus which may be used with that taught in these two patents to produce improved semiconductor ribbon material.
As taught in the above-referenced Jewett patent, various efforts have been made to grow monocrystalline ribbons of semiconductor material, such as silicon, directly from a molten mass of such material. If techniques for growing such materials can be perfected, the final cost of materials suitable for use in photovoltaic cells should be considerably less than that of wafers cut from monocrystalline boules grown by the Czochralski technique. Methods such as those taught by the two above-referenced patents have resulted in the growth, at least on an experimental basis, of semiconductor ribbons. However, these ribbons are generally formed of polycrystalline material having grains or crystallites with maximum dimensions ranging between 0.1 and 2.0 mm. Attempts to properly control melt temperature in the growth zone of horizontal systems such as that taught in the Jewett patent, have often resulted in propagation of dendrites which, in addition to not being a single crystal structure, cause rough surfaces and inhibit thickness control. Various prior art studies (see, for example, U.S. Pat. No. 4,256,681 issued to Lindmayer on Mar. 17, 1981) of polycrystalline silicon indicate that grains having a diameter of at least one millimeter are necessary to produce high efficiency solar cells. A smooth surface is, of course, necessary to allow processing of semiconductor ribbons into completed photovoltaic cells.